1. Field of the Invention
The present invention relates to switchable electrochromic devices that are capable of uniform switching throughout the entire structure. More particularly, the present invention is directed to switchable electrochromic devices, specifically aircraft window transparencies, which can uniformly switch between an activated and an unactivated state.
2. Technical Considerations
Commercial electrochromic devices are known in the art for use as transparencies, such as automotive windows, automobile mirrors, aircraft window assemblies, sunroofs, skylights, and architectural windows. Such electrochromic devices typically include a sealed chamber defined by two pieces of glass that are separated by a gap or space that contains an electrochromic medium. The electrochromic medium typically includes anodic compounds and cathodic compounds together in a solution. The glass substrates typically include transparent electrically conductive layers coated on facing surfaces of the glass and in contact with the electrochromic medium. The conductive layers on both glass substrates are connected to electronic circuitry. When the conductive layers are electrically energized, an applied potential is introduced into the chamber of the device, which electrically energizes the electrochromic medium and causes the medium to change color. For example, when the electrochromic medium is energized, it can darken and begin to absorb light.
Automotive windows, architectural windows, and some aircraft windows on the other hand, are relatively large in scale. As a result, switching between the lightened and darkened state in these larger scale electrochromic devices can be slow and non-uniform. Gradual, non-uniform coloring or switching is a common problem associated with larger scale electrochromic window assemblies, commonly referred to as the “iris effect”. This effect is typically due to the potential drop across the surface of the transparent conductive coatings present on the surfaces of the substrates, which results in the applied potential being highest adjacent to the bus bars along the edge of the surface coating and lowest at the center of the cell as the electrical current passes through the electrochromic solution. Accordingly, the electrochromic medium will typically display non-uniform coloring by initially coloring the perimeter of the cell where the bus bars are located, i.e., closest to the point where the applied potential comes in contact with electrochromic medium, and thereafter coloring toward the center of the cell. Moreover, in conventional electrochromic devices, the entire assembly is shaded upon application of electrical potential.
A need thus exists for electrochromic devices that are light weight, relatively inexpensive and durable.